Little is known regarding homing/engraftment and mobilization of hematopoietic stem and progenitor cells. We hypothesized that the CXC chemokine Stromal cell derived factor-1 (SDF-l/CXCL12) and its receptor CXCR4 are involved in these important and clinically relevant processes. Towards our long-term goals to modulate homing and mobilization for clinical benefit, we propose the following two Specific Aims: 1. Evaluate roles of SDF-1 (CXCL12) and its receptor, CXCR4, in the processes of stem and progenitor cell homing, engraftment and growth factor-induced or spontaneous mobilization in mice by modulating expression and activities of SDF-1 and/or CXCR4 in stem/progenitor cells and/or the microenvironment. To these goals, use the SDF-1 antagonist AMD 3100 as well as RSV-SDF-1 and LCK-SDF-1 transgenic mice to evaluate homing and engraftment, and AMO 3100 and G-CSF to evaluate mobilization in mice of differing genetic backgrounds as well as in RSV-SDF-1 and LCK-SDF-1 transgenic mice, and CCR1 -/- mice. 2. Evaluate mechanisms involved in SDF-1/CXCR4 effects on chemotaxis and mobilization of hematopoietic stem and progenitor cells in mice by determining intracellular signaling molecules and pathways involved in these effects in normal primary stem and progenitor cells, and by use of mice with functional deletions in selected intracellular signaling molecule. To the goals of aim 2, evaluate signaling in phenotypically defined populations of primary stem and progenitor cells using multivariate intracellular and cell surface flow cytometry, and use cells from mice functionally deleted in specific intracellular molecules or normal cells transduced with genes expressing dominant negative or activated forms of intracellular molecules. Also use intracellular gene deleted mice to assess their response to in viva mobilization with AMD 3100 and/or GCSF. These studies should clarity the relevance of SDF-1 and CXCR4 for homing/engraftment and mobilization of primary stem/progenitor cells.